Methods of preparing elastomers are well known in the art. In general, an elastomer is prepared by reacting a polyoxyalkylene polyether polyol with an organic isocyanate in the presence of a chain extender. The chain extender may be a diol or a mixture of triols and diols such that the overall functionality of the mixture is generally less than 2.3. The polyoxyalkylene polyether polyols used in the preparation of elastomers generally have molecular weights ranging from 2,000 to 5,000. For the preparation of sealants, the chain extender may be a triol or a mixture of diols, triols, and/or tetrols, such that the overall functionality of the mixture ranges from greater than 2.3 to 3.0.
Polyoxyalkylene polyether polyols used in the preparation of polyurethane elastomers are generally prepared by reacting an initiator compound with an alkylene oxide in the presence of a basic catalyst such as sodium hydroxide, potassium hydroxide, tertiary amine, or an alkoxide. Such catalysts are useful in the preparation of polyoxypropylene polyols until the equivalent weight of the polyol reaches about 1,000 to 1,200, at which point an excess of allylic terminal unsaturation starts to develop. Formation of unsaturation is believed to be a consequence of propylene oxide isomerizing to allyl alcohol, which subsequently reacts with propylene oxide to form allyl (2-propenyl)ethers of polyoxypropylene. The point at which unsaturation begins to develop and the rate of unsaturation can be influenced by variables such as, temperature, pressure, catalyst concentration, and the type of catalyst employed. Beyond certain equivalent weights, it becomes difficult, if not impossible, to make a polyoxypropylene polyether polyol having adequate functionality using conventional catalysts. Thus, as is discussed further below, many attempts have been made to solve the problem of unsaturation by varying the kinds of catalysts used in the preparation of the polyol.
The drawback of polyether polyols having high levels of unsaturation is that the allylic terminal unsaturation reduces the functionality of the polyol and terminates chain growth, in the final polyurethane, thereby reducing the polyol's equivalent weight and broadening its molecular weight distribution. Employing a polyether polyol with a less than anticipated functionality and a high level of unsaturation in the manufacture of polyurethane sealants and elastomers results in the degradation of mechanical properties, such as hardness, and tensile strength. While one may keep the level of unsaturation low by making a very low equivalent weight polyol, elastomers and sealants should be made with high equivalent weight polyols to enhance their elasticity. Therefore, it is highly desirable to manufacture a high equivalent weight polyether polyol, suitable in the manufacture of sealants, adhesives, and elastomers, which approximates the functionality of the initiator as close as possible.
Several attempts have been made to reduce the unsaturation of polyoxyalkylene polyether polyols by experimenting with the kinds of catalysts used in their preparation. For example, U.S. Pat. Nos. 5,136,010; 5,185,420; 5,266,681; 5,116,931; 5,096,993; 4,985,491 each disclose the preparation of polyether polyols using a double metal cyanide (DMC) catalyst to reduce the level of unsaturation down to about 0.04 meq/g of polyol or less. The disadvantages of using DMC catalysts to prepare polyols are that such catalysts are quite expensive; and, as reported in U.S. Pat. No. 4,355,188, the polyols containing the DMC catalyst residues are less stable during storage, may give an odor to the polyol, and causes undesirable side reactions during the manufacture of polyurethane products. In the manufacture of a block polyether polyol having an oxyethylene cap, it is usually necessary to remove the DMC catalyst used to prepare the block of oxypropylene groups prior to polymerizing the cap of ethylene oxide, because the DMC residual catalyst would prevent the uniform addition of ethylene oxide across all functional sites on the growing polymer. Thus, DMC must be removed and a standard catalyst, such as KOH, must be added as additional processing steps when polymerizing blocks of oxyethylene groups.
U.S. Pat. Nos. 4,902,834 and 4,764,567 describe the use of an alternative catalyst, cesium hydroxide, for reducing the unsaturation of polyoxyethylene polyether polyols. These patents, however, lack general teachings on how and what catalysts would be effective to reduce the unsaturation of polyoxypropylene polyether polyols. Furthermore, it would be desirable to manufacture a polyether polyol with its level of unsaturation not so dependent upon a specific catalyst.
In addition to double metal cyanide and cesium based catalysts for lowering the unsaturation of polyether polyols, U.S. Pat. Nos. 5,010,187 and 5,070,125 also describe the use of barium or strontium based catalysts for reducing unsaturation. As with the cesium and DMC catalysts described above, it would be desirable to manufacture a low unsaturation polyether polyol which is not catalyst-dependent.
U.S. Pat. No. 4,687,851 discloses a polyether polyol having an unsaturation level of 0.06 meq/g or less made with conventional tertiary amines or sodium and potassium hydroxides. To obtain the low unsaturation, the polyether polyol must be amine-initiated. There continues to exist a need for the manufacture of polyether polyols having a low degree of unsaturation which are not limited to a specific initiator and which can be manufactured in the presence of conventional or other low cost catalysts.
In this regard, we began to investigate lowering the degree of unsaturation through methods other than improving processing techniques or divising new catalysts. We went down a path not thought of as a means for lowering unsaturation. By altering the structure of the polyol molecule, we discovered that the degree of unsaturation can be lowered significantly no matter what kind of catalyst is employed.
The structure of polyether polyols can vary widely depending upon the desired application. For example, conjugated or block polymers of ethylene oxide and propylene oxide reacted onto an initiator molecule are known to impart unique properties in a particular application depending upon the order of oxide addition. U.S. Pat. Nos. 3,036,118 and 3,036,130 each disclose conjugated block polymers of polyether polyols having an internal oxyethylene block followed by a block of oxypropylene groups for use as nonionic surface active agents. U.S. Pat. No. 4,738,993 also discloses a polyether polyol having an internal block of oxyethylene groups usefull in the manufacture of RIM polyurethane elastomers. Polyether polyols having an internal block of oxyethylene groups have also found use in improving the air flow and load bearing properties of polyurethane foams, as disclosed in U.S. Pat. No. 4,487,854.
Reversing the order of ethylene oxide and propylene oxide addition is also known. For example, surface active, detergent, and anti-foam polyether polyols having an internal block of oxypropylene groups followed by a chain of oxyethylene groups are known according to the teachings of U.S. Pat. Nos. 2,674,619 and 2,948,757. Such polyols have also found use in the manufacture of flexible polyurethane foams according to U.S. Pat. No. 3,865,762.
Polyether polyols having a heteric structure, wherein a mixture of alkylene oxides are added onto the initiator molecule such that the oxyalkylene groups are distributed in a random fashion on each molecule, are also known according to the teachings of various patents. According to these patents, suitable alkylene oxides usually include ethylene oxide, propylene oxide, and butylene oxide. For example, U.S. Pat. No. 4,812,350 teaches the manufacture of a heteric polyether polyol having certain weight proportions of ethylene oxide and either butylene oxide and/or propylene oxide for use as an adhesion enhancer in covered polyurethane foam panels. U.S. Pat. No. 2,733,272 recommends using a heteric polyoxyethylene-polyoxypropylene polyether of glycerol as a lubricant, especially in brake fluids. Heteric polyether diols are also disclosed in U.S. Pat. No. 2,425,845, and U.S. Pat. No. 4,301,110 teaches the manufacture of a polyether polyol having a heteric structure of oxyethylene and oxybutylene groups, optionally capped with a block of oxyethylene groups, useful in the manufacture of reaction injection molded parts.
There also exist polyether polyols having both a heteric and a blocked structure. For example, U.S. Pat. No. 4,487,854 discloses a polyether polyol having an internal block of oxyethylene groups followed by a heteric mixture of ethylene oxide, butylene oxide, and/or propylene oxide, optionally followed by a block of oxypropylene or oxybutylene groups as a terminal cap. The polyether polyol is said to impart good air flow properties and load bearing properties to a polyurethane foam.
None of these patents, however, teach the concept of reducing allylic unsaturation by altering the structure of the polyether polymer, or how such alteration should be made to effect the lowing of the degree of unsaturation. Further, most of these polyether polyols are too hydrophilic to be useful in elastomer sealant and adhesive applications.